1. Field of the Invention
The present invention concerns a method and an arrangement for controlling printing by a thermotransfer printing device. The invention is used in apparatuses with relative movement between a thermotransfer print head and the print medium, in particular in franking machines, addressing machines and other mail processing apparatuses.
2. Description of the Prior Art
A franking machine with a thermotransfer printing device that allows an easy changing of the print image information is known from U.S. Pat. No. 4,746,234. Semi-permanent and variable print image information are electronically stored in a memory as position data and are read out to the thermotransfer printing device for printout. The print image (franking stamp image) contains postal information including the postal rate data for transport of the postal item, for example a postal value character image, a postal stamp image with the postal delivery location and date as well as an advertisement image.
The entire print image is printed in print image columns by a single thermotransfer print head in a manner controlled by a microprocessor. The columns are presented in an arrangement orthogonally to the transport direction on a moving postal item. The machine can achieve a maximum throughput of franking items of 2200 letters/hour with a print resolution of 203 dpi.
The franking machine T1000 that is commercially available from Francotyp-Postalia GmbH has only one microprocessor for control of a 30 mm-wide thermotransfer print head with 240 heating elements for printing in columns. All heating elements lie in a row disposed orthogonally to the transport direction. For printing, thermotransfer printers use an equally wide thermotransfer ink ribbon disposed between a surface to be printed (for example a postal item) and the row of heating elements. Each heating element includes an electrical resistor. At the resistor of the activated heating element, the energy of an electrical pulse is converted into heat energy which transfers to the thermotransfer ink ribbon. Printing necessitates melting a small area of an ink layer of the thermotransfer ink ribbon and adherence of the melted ink on the print medium surface. The printing ensues only when the heating element charged with the pulse has been brought to printing temperature, i.e. a temperature higher than the pre-heating temperature. Given movement of the thermotransfer ink ribbon simultaneously with the postal item relative to the heating element and ongoing heat energy supply, a line or dash is printed parallel to the movement or transport direction. A line is printed orthogonally to the movement or transport direction in a print column when all heating elements in the row of heating elements are simultaneously charged with respective electrical pulses for a predetermined, limited time duration (pulse duration). The pulse duration can be sub-divided into phases. A last phase (printing phase) in which the dots of a print column are printed exists within the predetermined, limited time duration (pulse duration). Additional phases of the activation of the heating elements in order to heat the heating element to printing temperature precede the last phase. The binary pixel data for activation of the heating elements of all print columns are stored in a volatile manner in a pixel memory. Given a low print resolution, the intervals of adjacent print columns are large and the binary pixel data of the print phase mirror the print image.
A longer single pulse can be divided into a number of pulses of equal pulse durations, each corresponding to a specific heating phase. Multiple phases are typically necessary in order to generate sufficient heat energy for melting a small portion of an ink layer under the heating element, to cause the melted ink to be printed on the surface of the postal item as a dot (DE 38 33 746 A1).
In principle, a high print resolution in each print phase can be achieved only when the activation of the heating elements for heating ensues in a timely manner in preceding phases. It should also be noted that the energy of an electrical pulse emitted to a heating element that is to be activated is likewise transduced at the resistor of the adjacent heating element in the row (heat conduction problem). The heat energy is reduced by cooling after the pulse has terminated. Due to the adjacent energy input by heat conduction, increasing the heat energy for the activation of specific heating elements in their heating phase may not be needed if sufficient heat energy is nevertheless present to effect melting of the ink layer area under that heating element. The microprocessor therefore also monitors and controls the energy distribution dependent on the pattern to be printed in addition to formulating and emitting binary pixel data for generation or non-generation of an electrical pulse. The original mirroring of the print image as binary pixel data is thereby suitably altered in the pixel memory so that a clean print image is created. This requires a comprehensive pre-calculation as is, among other things, known from DE 41 33 207.
A microprocessor with higher calculation speed could be used to achieve a higher print resolution. The output of binary pixel data to the thermoprinting head then would ensue more often per time unit during which a print medium is moved further by an equal portion of the transport path. The memory space requirement in the pixel memory, however, simultaneously increases due to the pixel data for each additional virtual column or heating phase. “Virtual column” means a further column in the print image that is not visible since it does not cause a dot to be printed in the heating phase.
The binary pixel data for activation of the heating elements in the printer of each printing column can be encoded into image information in a known manner and exist stored in the pixel memory in order to save storage space. A method for control of the per-column printing of a postal value character is known from EP 578 042 B1 (corresponding to U.S. Pat. No. 5,608,636), in which coded image information are converted into binary signals for activation of printing elements before each printing event. The converted variable and invariable image data are combined only during the printing. The decoding of the variable print data and provisioning of the print data for a complete column in a register thereby ensues via a microprocessor. Since the data for the next print column must be provided in the time between two print columns, calculation time of the microprocessor is required dependent on the amount of variable print data, the level of the franking item throughput and the print resolution. This increases the bus load and limits the possibility to print a franking stamp image faster on a franking item.
The processing burden on the microprocessor can be relieved by hardware for print control. A device and a method for per-column printing of an image in real time is known from U.S. Pat. No. 5,651,103 in which variable and fixed image data elements are connected with one another and stored in a buffer in order to then be used for printing a column. The variable and fixed image data elements are stored in a non-volatile memory, wherein a portion of the fixed image data elements is compressed. The print image data are assembled from variable and invariable image data by the hardware for the printing of each print column only just before its printing, meaning that the image data for a printing event do not exist in binary form in a memory area but instead exist in an encoded form comparable to the method disclosed for the T1000 in EP 578 042 B1. The variable image data elements in the non-volatile memory are identified by a controller, and data that correspond with the variable image data elements are transferred to the hardware in order to download the variable and fixed image data elements, to connect them with one another and then to print them. The hardware for this purpose requires a variable address register for each variable image data element. The number of the variable image elements is thus limited by the number of the address registers.
Since the commercial introduction of the franking machine T1000 in 1991 by Francotyp-Postalia, which allowed (for the first time) the aforementioned advertisement stamp image to be electronically changed at the touch of a button in addition to the date and the postal fees, the requirements for its microprocessor controller have continuously grown larger. The more data that are processed, the more variable data are required in the print image. It is also necessary to generate other print images that substantially differ in design and content from a franking stamp image in order, for example, to print out business cards, fee stamp images and legal expense stamp images. The requirements for the print resolution in dots per inch (dpi) continuously increase. Given the printing of a dot, the aforementioned heat conduction problem between the adjacent heating elements due to the pixels adjacent to the print image to be printed becomes even more significant the closer the adjacent pixels are. This problem associated with thermotransfer printing, increases at high print resolution.
Modern franking machines should enable printing of a security imprint, i.e. an imprint embodying a special marking in addition to the aforementioned information. For example, a message authentication code or a signature is generated from the aforementioned information and then a character string or a barcode is/are embodied in such a marking. When a security imprint is printed with such a marking, this enables a verification of the validity of the security imprint, for example in the post office or at a private carrier (as described in U.S. Pat. Nos. 5,953,426, and 6,041,704).
In some countries, the development of the postal requirements for a security imprint has had the consequence that the quantity of the variable print image data that must be changed between two imprints of different franking stamp images is very high. For example, for Canada a data matrix code of 48×48 image elements must be generated and printed for every single franking imprint.
For more rational postal distribution and to increase security against counterfeiting, a new standard called FRANKIT was introduced in 2004 in Germany by the Deutschen Post AG. In response, some franking machines the print resolution is increased by the use of a postal ½-inch inkjet print head with bubble jet technology that is arranged in a cartridge and is secured by suitable means (EP 1 132 868 A1).
A FRANKIT-compatible franking machine Ultimail® 60 is commercially available from Francotyp-Postalia that uses two modified 600 dpi inkjet print heads to generate a security imprint with 300 dpi print resolution (FIG. 1).
An arrangement to control printing in a mail processing device is known from EP 1 378 820 A2 (corresponding to U.S. Pat. No. 6,733,194) that has a print data controller for pixel data preparation during the printing with a print head. The print data controller is connected with a pixel memory via a bus. The circuit arrangement includes a DMA controller, a printer controller, as well as at least one pixel data preparation unit with two buffers for per-data string data transfer from the pixel memory, the two buffers are alternatingly written with data and read out. The aforementioned circuit arrangement, however, is not suitable for a controller for a thermotransfer printing device. In order to achieve a FRANKIT-compatible franking machine with thermotransfer printing, the print data controller that relieves the processing burden on the microprocessor would have to be modified. For faster printing at high print resolution, however, additional encoded pixel data would still have to be stored in columns in the pixel memory and transferred in succession into a printer controller for all phases preceding the print phase, whereby virtual columns are temporally situated between the print columns and contain encoded pixel data which serve for pre-heating of the heating elements. For example, if pixel data were stored and transferred as valid voltage values per pulse duration, a significant storage requirement would result in the machine as well as a correspondingly high time requirement for the transmission of such data.